We had a similar problem with our 96 Holiday Rambler Imperial. Charging voltage was around 15 volts and the battery light would flash. It only went out when I put a load on it with headlights, heater motor,etc. Records showed that the previous (original) owner had replaced or repaired the alternator seven times at a cost of more than three thousand dollars. The alternator is a Leece Neville, internal regulator with external excitation.
After chasing several wrong theories, I found that the alternator input from ignition (excitation) was hot all the time while the battery lead (voltage sensing) was hot only with the ignition switch on. The installed condition was in accordance with color coding according to wiring schematics provided by Spartan. It turns out that the leads were mistakenly grafted inside of the wiring harness. Switching the leads fixed the problem.
I don't know what your coach is, but if chassis is Spartan it might be worth checking. If you need to swap the leads, be careful. It is easy to mess up the terminal posts on the back side of the alternator (and I'm not saying how I know). Good luck. Let me know if I can help.